Looping through all nodes of a content type and copying one textfield of the node into the body field

One way of approaching your problem is to use Views Bulk Operations to update your nodes. Views Bulk Operations allows you to run customized actions (such as Rules' action components) on selected nodes.

A brief outline:

• Add an action set in Rules, copying the text field value to the body. (You will need the set data value action for this.) Make sure that the action set has one argument/parameter – a node.
• Build a view listing all nodes where the body field is empty.
• Add a VBO node action field to your view. Select the action set you built.
• Visit your view, check the nodes you want to update (such as all of them), and run the action.

Good luck!
//Johan Falk
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You might also use the database itself:
UPDATE node_revisions, content_field_article SET node_revisions.body = content_field_article.field_article_value WHERE node_revisions.vid = content_field_article.vid;

http://drupal.org/node/165370